1. Field of the Invention
This invention relates to a process for producing hypophosphorous acid (H.sub.3 PO.sub.2) and non-transition metal hypophosphites therefrom, and particularly relates to converting byproduct phosphine produced from certain processes, to hypophosphorous acid (H.sub.3 PO.sub.2) and hypophosphites therefrom.
Alkali metal hypophosphites, and in particular, sodium hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O) have been used for many years in medicine. Originally, hypophosphites, were combined with cod liver oil and other ingredients (Scotts emulsion) and made into various syrups. A relatively recent application of hypophosphites is in the chemical reduction of nickel salts to form smooth, adherent platings. This commercial plating process, which is superficially similar to the silvering of mirrors, has been called "electroless nickel plating" (ENP). Such plating is used for protecting the interiors of large vessels, such as tank cars and provides better wear and corrosion resistance than other similar type coatings, e.g. chrome plating. Chrome plating processes are now particularly disfavored, due to the environmental concerns related to the toxicity of effluents from such processes which contain chromium. Other electroless nickel plating applications are in printing rolls, baking pans and processes for the recovery of oil.
In commercial processes for producing hypophosphites, for example, sodium hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O), phosphorus, and in particular, yellow phosphorus (P.sub.4), is reacted with an aqueous metal hydroxide composition, typically an aqueous suspension of sodium hydroxide and calcium hydroxide, to produce the hypophosphite. Typically, about 50% of the phosphorus is converted to the hypophosphite, about 25% is converted to phosphite, which is precipitated, for example, as a calcium salt, and the remaining 25% is converted to phosphine (PH.sub.3), (see, for example, Auslegeschrift No. 1,112,054 to Knapsack-Griesheim A.G.). Generally, the phosphine is oxidized to P.sub.2 O.sub.5 and H.sub.2 O by burning. This then forms phosphoric acid. This procedure for disposing of phosphine is economically wasteful and can cause pollution problems if appropriate pollution control equipment is not utilized.
There have been attempts to convert phosphine to hypophosphites (H.sub.2 PO.sub.2.sup.-), for example, by reacting phosphine with sodium hypochlorite in an alkaline solution. This process is unsatisfactory because it produces byproduct sodium chloride which is difficult to remove from the product stream (see, for example, Lawless and Searle, "Kinetics of the Reaction Between Phosphine and Sodium Hypochlorite in Alkaline Solution", J. Chem. Soc. 1962, 4200-5).
2. Prior Art
It is known that hypophosphorous acid (H.sub.3 PO.sub.2) may be converted to sodium hypophosphite by reaction with sodium hydroxide. It is also known that phosphine can be converted to hypophosphorous acid by passing the phosphine into an aqueous suspension of iodine until it is colorless and repeatedly distilling to remove the hydrogen iodide to leave pure hypophosphorous acid. (See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 15, Second Completely Revised Edition, 1968). It is also known that phosphine may be oxidized in the presence of chlorine (Chem. Abst., Vol. 76, 1972, 63977p).
It is also known that phosphine can react with hydrogen peroxide in the presence of catalysts such as potassium iodide (C.A., Vol. 76, 1972,145334r), bromide ions (C.A., Vol. 76, 1972,50585j), copper (II) (C.A., Vol. 84, 1976,65680a), certain iodide and bromide iron complexes (C.A., Vol. 82, 1975,160755s), and iron (III) bromide and lithium chloride (C.A., Vol. 82, 1975,48123n).
A process to selectively convert byproduct phosphine (which, for example, is produced from reacting phosphorus and an aqueous metal hydroxide solution and contains hydrogen) to hypophosphorous acid and then to sodium hypophosphite through any of these aforementioned catalytic routes would not be satisfactory since such catalytic processes tend to produce large amounts of phosphorous acid (H.sub.3 PO.sub.3) and phosphoric acid (H.sub.3 PO.sub.4) rather than hypophosphorous acid (H.sub.3 PO.sub.2), i.e. the catalytic processes tend to oxidize the phosphine to too great an extent. Additionally, the catalyst tends to decompose the hydrogen peroxide to water and oxygen. Such decomposition is economically undesirable and presents a hazard.
Paris and Tardy, COMPT. rend 223 (1946) 242/3 indicates that phosphine can be oxidized by hydrogen peroxide to hypophosphorous acid in the absence of a catalyst. Z. Anorg. Allgem. Chem. 121 (1922) 73194, 75 and Sokol 'Skii. D. V., et al. (USSR), Gomogen Kataliz, 1970, 79-89, abstracted in C.A., Vol. 76, 1972, 145334r indicates that phosphine cannot be oxidized by hydrogen peroxide in the absence of a catalyst. Thus, there is confusion by those skilled in the art whether phosphine can be selectively oxidized by hydrogen peroxide in the absence of a catalyst and, if so, under what conditions.